1. Field of the Invention
The present invention relates to a method for manufacturing a highly-crystallized metal oxide powder, semimetal oxide powder, or double oxide powder comprising at least two metal elements and/or semimetal elements. In particular, the present invention relates to a method for the manufacture of an oxide powder with a high dispersibility and a high crystallinity that has a high purity and a uniform particle size, this powder being suitable for functional materials for electronics such as phosphor materials, dielectric materials, magnetic materials, conductive materials, semiconductive materials, superconductive materials, piezoelectric materials, magnetic recording materials, positive electrode materials for secondary batteries, and electromagnetic wave absorbing materials, catalyst materials, and starting materials for the manufacture thereof, or for industrial materials that can be used in a variety of other fields.
2. Description of the Prior Art
Powders of metal oxides, semimetal oxides and double oxides comprising at least two metal elements and/or semimetal elements (those powders will be collectively referred to hereinbelow as “oxide powders”, unless stated otherwise) employed as functional materials are desired to have a high purity, to be compositionally homogeneous, and to have a high crystallinity in order to demonstrate fully their functions. In particular, in order to improve phosphor characteristics such as fluorescence intensity of phosphors, a highly-crystallized oxide powder is required, this powder containing a small amount of impurities, having no defects or lattice deformations on the surface of particles or inside thereof, being compositionally homogeneous, in particular, having a very small amount of activation elements uniformly distributed therein, and preferably consisting of a single phase.
When an oxide powder is molded and heat treated by a sintering process to manufacture a sintered body, property control of the oxide powder as a starting material is also important. For example, in order to obtain high-performance oxide cores or oxide permanent magnets with excellent magnetic properties and mechanical properties, the oxide powder used as a starting material is required to consist of fine particles, to have a uniform particle size and an isotropic shape, and to be a single crystal.
Furthermore, when an oxide powder is dispersed in a matrix such as a resin or the like and used in the form of a thick-film paste, ink, paint, sheet, powder compact, or other compositions and composite materials, it is important that in addition to the improvement of properties inherent to the oxide, the particle shape and size be uniform and agglomeration of particles be prevented in order to improve dispersibility, packing density, and processability. In particular, a fine monodisperse powder with a mean particle diameter of about 0.1–10 μm, a narrow particle size distribution, and no agglomeration is desired for thick-film pastes or inks.
Oxide powders have been manufactured in the past by a solid-phase reaction method, a gas-phase reaction method, a liquid-phase reaction method, and a spray pyrolysis method.
With the solid-phase reaction method, a mixture of starting material powders such as oxalates, carbonates, oxides, and the like is placed in a firing container such as a crucible or the like and heated at a high temperature for a long time to induce a solid-phase reaction, followed by grinding in a ball mill or the like. However, the oxide powder manufactured by this method has an irregular shape and consists of particle aggregates with a large particle size distribution. Moreover, a large amount of impurities are introduced from the crucible or in the grinding process. Further, when double oxides are manufactured, treatment is required to be conducted for a long time at a high temperature in order to improve compositional homogeneity. As a result, the production efficiency is poor. Furthermore, the particle surface was modified and a large number of defects were produced on the particle surface and inside thereof by chemical reactions and physical impacts during grinding. As a result, the crystallinity was decreased and physical properties inherent to oxides were degraded.
With the gas-phase method by which vapors of metals or metal compounds are reacted in a gas phase, fine oxide powders can be manufactured. However, not only the cost is high, but agglomeration easily occurs in the obtained powder and the particle diameter is difficult to control.
Examples of the liquid-phase reaction method include a liquid-phase precipitation method, a hydrothermal method, and a method based on hydrolysis of inorganic salts or alkoxides. Those methods produce fine oxide powders with a comparatively small surface modification and a high crystallinity. However, fine powders without agglomeration and with a high dispersibility are difficult to manufacture. Furthermore, high-purity starting materials are necessary and a long time is required for the reaction and separation operation, which results in a high production cost.
With the spray pyrolysis method, a solution obtained by dissolving or dispersing a metal compound in water or an organic solvent is sprayed to obtain fine liquid droplets and the droplets are heated under the conditions allowing a metal oxide to precipitate, thereby producing a metal oxide powder. With such a method, agglomeration-free fine monodisperse particles can be obtained and the amount of introduced impurities is small. Furthermore, because the starting materials are in the form of a solution, metal components can be mixed homogeneously at any ratio. For those reasons, this method is considered to be suitable for the manufacture of double oxide powders. For example, Japanese Patent Publication No. 2001-152146 described the manufacture of a fine phosphor powder with excellent fluorescence properties by this method.
However, the spray pyrolysis method uses a large amount of water or an organic solvent such as an alcohol, acetone, an ether, and the like to obtain liquid droplets of the metal compounds used as a starting material. For this reason, a large quantity of energy is required to evaporate the solvent, energy loss during pyrolysis is increased, and the cost rises. Furthermore, the atmosphere control during pyrolysis is difficult because of solvent decomposition. In addition, the particle size distribution of the produced particles sometimes becomes wide because of merging and splitting of liquid particles in the reaction vessel. For those reasons, the reaction conditions such as the spraying rate, the concentration of liquid droplets in a carrier gas, the retention time in the reaction vessel, and the like are difficult to set and the productivity is poor. Moreover, this method is limited to those starting materials that can be used to obtain solutions or suspensions. Therefore, a limitation is placed on the compositional range and concentration of starting materials, and the types of oxide powders that can be manufactured are limited.